1. Field of the Invention
This invention relates to an optical wavelength conversion element which converts a fundamental wave to a second harmonic, and more particularly to an optical wavelength conversion element in which periodic domain reversals are formed on a ferroelectric material having a nonlinear optical effect. This invention further relates to a method of manufacturing such an optical wavelength conversion element having domain reversals.
2. Description of the Related Art
There has been proposed by Bleombergen and et al. a method of converting a fundamental wave to a second harmonic by use of an optical wavelength conversion element formed with a region where the spontaneous polarization (domain) of a ferroelectric material having a nonlinear optical effect is periodically reversed. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)) In this method, by setting pitches .LAMBDA. of the domain reversals to an integer multiple of the coherence length .LAMBDA.c given by formula EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
wherein .beta.(2.omega.) represents the propagation constant of the second harmonic and .beta.(.omega.) represents the propagation constant of the fundamental wave, phase matching (artificial phase matching) between the fundamental wave and the second harmonic can be obtained. When the wavelength is converted by use a bulk crystal of a nonlinear optical material, phase matching can be achieved only at a particular wavelength inherent to the crystal. However in accordance with the method described above, phase matching can be efficiently achieved for any wavelength by selecting pitches .LAMBDA. of the domain reversals to satisfy the formula (1).
As a ferroelectric material suitable for forming such periodic domain reversals, there has been known LiNbO.sub.3 doped with Mg as disclosed, for instance, in U.S. Pat. No. 5,568,308. Mg-doped LiNbO.sub.3 is higher than non-doped LiNbO.sub.3 by more than two digits in the optical damage threshold value. Accordingly when periodic domain reversals are formed on Mg-doped LiNbO.sub.3, an optical wavelength conversion element which can generate a high power wavelength-converted wave with a high wavelength conversion efficiency can be obtained.
As another ferroelectric material suitable for forming periodic domain reversals, there has been known LiTaO.sub.3 doped with Mg. There have been made various attempts making optical waveguide type or bulk crystal type optical wavelength conversion elements by use of such ferroelectric materials.
As a method of forming periodic domain reversals on a ferroelectric material, there has been known a method in which periodical electrodes each having a predetermined width are formed on a ferroelectric substrate at predetermined pitches and electric fields are imparted to the ferroelectric substrate through the periodical electrodes as disclosed in U.S. Pat. No. 5,568,308.
However the conventional optical wavelength conversion elements comprising a substrate of Mg-doped LiNbO.sub.3 or Mg-doped LiTaO.sub.3 formed with periodic domain reversals are disadvantageous in that the pitches of the periodic domain reversals are apt to fluctuate and it is difficult to achieve a high wavelength conversion efficiency.
Further in the conventional optical wavelength conversion elements, each of the periodic domain reversals is apt to be formed wider than the width of each electrode though it should be equal to the width of each electrode. Thus there has been a problem that it is difficult to form each of the periodic domain reversals precisely in a desired width.